1. Field of the Invention
This invention relates to a brushless motor.
2. Description of the Prior Art
A brushless DC motor utilizing a rotor composed of a permanent field magnet, an armature functioning as stator and a position detecting element for detecting the position of magnetic poles of said permanent magnet rotor and accordingly switching the electric current to said armature stator thereby causing rotary motion of said rotor is already disclosed for example in U.S. Pat. No. 3,548,224. In the brushless DC motor disclosed in the above mentioned patent there is provided a coil holding element in such a manner to enclose a permanent magnet rotor, which coil holding element is provided with armature coils thereon which are supported by an annular stationary yoke composed of a smooth iron core provided around said coils, and there is also provided a position detecting element in a place on said stator so as to detect the position of magnetic poles of said permanent magnet rotor through said coil holding element and said armature coil. Such motor, due to the structure thereof in which the permanent magnet rotor is enclosed in the coil holding element and the armature coils constituting a part of stator, is unable to permit separate manufacture of stator and rotor followed by simple assembly of the thus prefabricated stator and rotor, but inevitably requires sequential steps of placing the coil holding element around the permanent magnet rotor, then winding the armature coil on said coil holding element and so on, and thus allows only a very poor production efficiency. Also, the rotor is generally supported rotatably by the shaft thereof in the bearings of the stator, and high speed rotation usually required in small-sized motors results in wear in a relatively short period in said bearings provided on the stator, or in said shaft at the parts thereof in contact with said bearings. Since the rotation accuracy of the motor is significantly affected by such wear, frequent replacement is inevitable for such motors when used in precision instruments. In such case it is a general practice to replace not the whole expensive motor but the worn parts, i.e. the bearings on the stator or the rotor. For example, if the rotor shaft is worn out, the worn rotor is extracted and replaced by a new rotor, and the precision of rotation is tested. However, the motor structure shown in U.S. Pat. No. 3,548,224 wherein a permanent magnet rotor is enclosed in a coil holding element on which certain turns are wound is not capable of accepting such dismantling and reassembly, as the replacement of rotor or stator alone inevitably requires the steps of dismantling the armature coil, removing the coil holding element, reassembling the coil holding element around the replaced rotor and rewinding the armature coil. Further, the position detecting element, detecting the positions of magnetic poles of the permanent magnet rotor through said coil holding element and armature coil, receives very little magnetic flux from said permanent magnet rotor and thus is unable to provide satisfactory electronic control.
In order to prevent the difficulty of dismantling and reassembly in a brushless DC motor wherein a permanent magnet rotor is enclosed in a coil holding element on which is provided an armature coil, there is already proposed, in U.S. Pat. No. 3,488,566, a motor structure employing an armature coil of a drug-cup shape with one open end in which a permanent magnet rotor is placed. In such a brushless DC motor, the accuracy of rotation and the torque are significantly dependent on the structure of the armature coil and, in order to improve these properties, it is necessary to increase the magnetic flux density by reducing the length of the magnetic gap or by increasing the number of armature coil turns. Also, as to the shape of said armature coil, the radial dimension and the thickness thereof have to be maintained constant or otherwise there will result an unbalanced distribution of magnetic flux which will inevitably necessitate tedious adjustments at the assembly of motor. Thus, such an armature is not suitable for mass production and is unable to provide a satisfactorily high efficiency. Further, as the efficiency of an armature coil improves as the thickness thereof decreases, it is vitally necessary in such a brushless DC motor to provide as many coils as possible without widening the gap in the magnetic circuit, thereby increasing the magnetic flux density in said gap. However, the above mentioned U.S. Pat. No. 3,488,566 merely discloses a structure of an armature coil of a drug-cup shape with one open end in which a permanent magnet rotor is placed, and does not show the position of the position detecting element for detecting the positions of magnetic poles of the permanent magnet rotor, which is extremely important for realizing accurate rotation, nor does it provide any description as to the structure of the armature coil body adapted to increase the magnetic flux density. Besides, the armature coil disclosed in said patent has a thickness at the connecting portions at the open end thereof which is larger than the remaining part, and the rotor is placed in said armature coil and at a position so as not to collide with said thicker part, thereby preventing an unbalanced structure of the rotor. As a result, the thicker part extends outwardly from said rotor, thus increasing unnecessarily the axial dimension of the motor.